I purchased a electric tankless water heater and I would like to know the following? The unit requires 3 separate 40 amp breakers, one for each line. I have 200 amp service. The subpanel will be located about 30 feet from the main. What would the breaker be at the main and what size wire should I run from the main to the sub-panel.

  • Where do you live? Three separate lines (3-phase) is pretty uncommon in homes (US anyway). I think you answered your first question; 40A right? 30' isn't enough to have a worthy voltage drop. Speaking generally, #8 wire's good for 40A. Feb 5, 2015 at 6:56
  • What is the make and model of the heater? When installing equipment like this, the manufacturers installation instructions must always be followed. To answer this question properly, we'll need to read them.
    – Tester101
    Feb 5, 2015 at 11:34
  • 1
    @ChiefTwoPencils, we can safely assume he means it requires three 40A/240V circuits. This is not uncommon. Mar 7, 2015 at 15:57
  • 3
    @Ron, did you do a load calculation to see if your current service can even handle this additional load??? Mar 7, 2015 at 15:58
  • @SpeedyPetey ah, I see. Never installed one, but apparently my plans to have one installed are off the table with that load. Mar 7, 2015 at 16:56

2 Answers 2


I'm not sure where you live but US code calls for a minimum of #8 if you're using copper or #6 if you're using aluminum. There would be no advantage in using a larger wire size in this case because at 30ft you will have negligible losses.

I don't know what your voltage is or how many phases you have but for an example of what your losses may be, I will assume a worst case scenario - 100V single phase. If you used the #6 aluminum wire, you would have a maximum of 1.72% voltage drop (loss). If you used the copper #8, you would have a maximum voltage drop of 1.66%. And just remember, when you are calculating losses (voltage drop), the higher the voltage, the better because higher voltages have less voltage drop.

Note: I use the Southwire Voltage Drop Calculator for most of my voltage drop calculations. http://www.southwire.com/support/voltage-drop-calculator.htm

  • At 30' figuring voltage drop is pretty much unnecessary. Besides, what are you basing these numbers on? Not 40A each I hope. Mar 7, 2015 at 16:00
  • @SpeedyPetey Yes, that was my point. I was saying that at the smallest legal wire size, the voltage drop would be negligible. Yes, I was taught in the Solar School I attended to base the voltage drop off of the breaker size in this situation for two reasons. First, the actual amperage is not listed. Second, you never know what someone else may put on the same circuit. However, you said "At 30' figuring voltage drop is pretty much unnecessary." I would like to mention that while this is mostly true, if you are working with low voltage (such as 12V) that voltage drop is very necessary @ 30'. Mar 9, 2015 at 2:54
  • 1
    Using the breaker size for voltage drop can yield some erroneous numbers, and some costly ones as well. You would typically never use the actual circuit size when figuring VD. Also, we are NOT talking about 12v circuit here. Even if so, 30' is still borderline as to being an issue. Mar 9, 2015 at 19:41

Use #2/3 copper with ground to a 115 or 125 amp single phase subfeeder panel fed with a 2 pole 115 amp breaker. Three 40 amp double pole breakers using 8/2 copper with ground feeding the instant hot from the subfeeder panel.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.